Many electronic devices require a constant DC source in order to operate. The power to the electronic device can initially be supplied from a DC power source, such as from a DC-to-DC converter that receives its DC input from an AC-to-DC converter. Often, two separate power supplies are used when applications have an AC universal input requirement and a low voltage DC input requirement. For example, an application could require an AC universal input requirement where an AC voltage is converted to a DC output. This AC converter could have a range of AC input voltages. This, in turn, would create a range of DC voltage output values corresponding to the rectified voltage obtained from a range of AC input voltages. This is often known as the bulk voltage corresponding to the range of DC voltage that is produced by the AC input voltage. For example, a transformer could put out 150 to 400 volts. The nominal voltage is the DC voltage that is within the range of the DC bulk voltage produced by the AC input voltage. There could also be a low voltage DC input requirement, which is lower than the range of DC bulk voltage produced by the AC input output. The bulk voltage is that DC voltage that is obtained from the AC rectification. This DC voltage is lower than the range of the DC voltage produced by the AC input voltage. Typically, two separate power supplies are used, typically, one with a universal AC input and another DC-to-DC converter.
U.S. Pat. No. 5,126,652 to Carlin is directed to a power supply that is designed to operate over a wide range of both AC and DC input voltages and frequencies from 0 to 400 HZ. It can provide two regulated low voltage DC outputs that are based on accepting input voltages ranging from 12 to 240 volts DC or 24 to 240 volts AC. A high voltage n-channel MOSFET transistor controls the high voltage primary switching transistor. This device has a circuit that protects that transistor by providing a power-up in-rush clamping feature in a power supply circuit. This is accomplished by a bypass diode and a dynamic feedback circuit, which limits the switching current to approximately two times the nominal switching current.
However, the device of the '652 patent would not allow a single power supply to provide the desired isolation and operation from a full range of common input AC names and low (and nominal) DC input voltages.